Ethylene copolymers having excellent optical properties in film form can be prepared, as described in European patent publication No. 4647, by continuously copolymerizing ethylene with one or more higher alpha olefin monomers via a low pressure gas phase reaction with a catalyst composition prepared by (1) forming a precursor composition from a magnesium compound, titanium compound and electron donor compound, (2) impregnating said precursor composition in a particulate, inert, porous support, and (3) activating the precursor composition with an organoaluminum compound. While films formed from such copolymers generally have good clarity, they are characterized by visual imperfections known as "pinstriping" and "gel streaking". By "pinstriping" is meant haze bands which are produced when the copolymer is extruded into film as accumulations of polymeric micro-gels in the copolymer are aligned in the film by the shear forces exerted during the extrusion. By "gel streaking" is meant lineations in the surface of the film, sometimes having a V or chevron shape, which are caused by the flow of polymeric gels into the molten polymer surface during extrusion. In extreme cases, these lineations can completely permeate the film. Both these phenomena not only detract from the appearance of the film, but also weaken the mechanical strength of the film.
Both pinstriping and gel streaking are known to result from the presence of polymeric gels in the copolymers. Such gels usually develop during processing of the copolymers into film. In order to prevent undesirable polymer degradation during such processing, as well as during storage, it is common to add a hindered phenolic anti-oxidant to the copolymers before attempting to extrude them into film. However, at processing temperatures of about 210.degree. C. or above, residual halogen from the catalyst employed in the preparation of the copolymers reacts with copolymer melt to produce hydrogen halide gas, which hydrogen halide gas in turn interacts with the phenolic anti-oxidant and the copolymer. This latter reaction produces the gel formation which is responsible for the pinstriping and gel streaking effects created when the copolymer is extruded into film.
In addition to producing films characterized by pinstriping and gel streak defects, it has been found that the copolymers produced as described in European patent publication No. 4647 often cause corrosion of the molding and extrusion equipment employed to process them. This corrosion results from the liberation of hydrogen halide gas during processing of the copolymers at temperatures of 210.degree. C. or more.